Coaxial cables are formed from an inner conductor and an outer conductor concentrically disposed around the inner conductor, with a nonconducting insulation being uniformly disposed therebetween. Coaxial cables are used to carry radio frequency or microwave frequency electrical signals.
Coaxial connectors are employed to join one coaxial cable to another or to join a coaxial cable to an appropriate electrical circuit such as a circuit board, a microstrip, a coplanar wave guide or the like. The coaxial connector enables the structural and electrical mating of the outer conductor of the socket to the outer connector of the plug, and also the inner connector of the socket to the inner connector of the plug. The inner conductor of the coaxial connector typically is of very smaller diameter and extends a longitudinal distance that is great compared to the small diameter. For example, a widely accepted standard for sub-miniature coaxial connectors (SMA connectors) provides a male pin having a diameter of 0.036-0.037 inch. The female socket to which this male pin will be mated typically will have an external dimension of only 0.0495-0.0505 inch. In view of these relatively small dimensions, it is necessary to provide a support between the inner and outer conductors of both plug and socket members to assure the coaxial symmetry of each member. This support typically is provided by a non-conducting, insulating material, such as a plastic bead which is concentrically disposed between the inner and outer conductors and which provides the necessary concentric support therebetween. The plastic bead typically is formed form a dielectric material such as polyphenylene oxide. The dielectric support bead must be precisely manufactured to be nearly perfectly concentric relative to the inner and outer conductors. Any nonconcentricity or any burrs, nicks or the like which may occur during manufacturing will invariably affect the electrical performance characteristics of the connector by causing losses to occur under certain operating conditions or at certain frequency ranges. Similar problems will occur if the dielectric material from which the bead is formed is of nonuniform density throughout.
All dielectric support beads create an impedance regardless of the precision with which they are manufactured. This impedance is proportional to the dielectric constant of the support bead. The dielectric constant, in turn, is inversely proportional to the radio frequency at which the connector is capable of operating. More particularly, a support bead having a low dielectric constant will be able to operate at higher frequencies. The typical prior art SMA coaxial connector operates at or below 26 GHz.
There have been ongoing attempts to develop coaxial connectors that can reliably carry frequency signals greater than 26 GHz. Furthermore, there is a particular desirability of providing a coaxial connector that can perform at very high frequencies and that is both mechanically and electrically compatible with the widely used SMA connectors. In particular, it would be desirable to produce a connector that is compatible with SMA connectors and that can reliably accommodate frequencies up to and above 40 GHz and that also will match or exceed the electrical performance of the standard SMA connectors at lower frequencies, such as frequencies below 26 GHz, as well as the mechanical performance of the standard SMA connectors.
The dimensions of the standard SMA connector severely limit the options that are available for developing a dimensionally compatible connector with much higher frequency limits. More particularly, since the dimensions of any new high performance connector must be compatible with the dimensions of the known SMA connectors the only significant improvements in performance can be achieved by providing an improved dielectric bead and by providing a high frequency connector that will be less susceptible to contamination. Improvements in the performance of the dielectric bead would require decreasing the dielectric constant of the support bead so that higher frequencies can be obtained. This objective, however, cannot easily be achieved because the support bead of an SMA connector typically has a diameter of 0.160 inch or less and an average length of approximately 0.235 inches. Furthermore, any structure to reduce contamination within the connector, such as contamination caused by moisture or by the flaking of the gold plated members of the connector, presumably would adversely affect the performance of the connector.
Recently, it has been theorized that the dielectric constant of the support bead could be decreased by providing a generally symmetric array of longitudinally aligned apertures through the bead, thereby reducing the amount of dielectric material and thus reducing the overall dielectric constant. This decrease in the dielectric constant would in theory enable the connector to operate at higher frequency levels. In operation, it was believed that the dielectric bead could be manufactured from an elongated extruded rod of the selected plastic dielectric material. The rod would first be drilled with a central aperture, and the free end of the rod would be machined with an annular channel. The length of the rod would then be machined down to the required outside diameter of the dielectric bead. A section of the rod would then be cut off to the required length of the dielectric support bead, and an annular channel would then be machined into the opposed end of the bead, with the dimensions of both annular channels being selected to create compensation steps required to compensate for the stepped configurations of portions of the inner or outer connector body in which the dielectric bead is mounted. Finally, a plurality of longitudinally extending apertures would be drilled entirely through the support bead between the central aperture and the outer circumference. Attempts to manufacture support beads of this type at a production scale proved entirely unacceptable. More particularly, it was found that variations in the temperature at the extrusion head which produced the rod from which the beads would be formed resulted in slightly different densities along the length of the rod. Consequently, the impedance characteristics often would differ from one bead to the next or would be variable across any given bead. Additionally, all of the plastic materials from which the support bead might be formed are somewhat resilient and cannot be manufactured at production rates to the required accuracy, and furthermore such plastic materials have a "memory" thereby possibly resulting in distortion of the resulting bead when machined during manufacturing. These problems inherent to plastics were exacerbated by the very small size of the bead and by the small webs existing between the apertures formed in the bead. As a result, the beads would structurally vary from the specifications with corresponding degradations in anticipated performance.
Additionally, the bead described above did not adequately protect against microscopic contamination, such as contamination from gold flakes or moisture. This contamination could offset a substantial part of the improvements that were believed to be attainable with the suggested configuration. Extensive inquiries to manufacturers of plastic components indicated that the machining manufacturing process, with its various drawbacks, was the only option in view of the small size, the material requirements and the very high precision required for the bead dimensions, corners and apertures.
In view of the above, it is an object of the subject invention to provide a high frequency coaxial connector that can efficiently perform at frequencies up to or above 40 GHz.
Another object of the subject invention is to provide a high frequency coaxial connector that is mechanically and electrically compatible with existing sub-miniature standards for coaxial connectors.
A further object of the subject invention is to provide a support bead for a high frequency coaxial connector capable of efficiently operating at frequencies up to 40 GHz.
An additional object of the subject invention is to provide a molded support bead for a high frequency coaxial connector having a plurality of apertures extending longitudinally at least partly therethrough.
Still a further object of the subject invention is to provide a high frequency coaxial connector with a substantially reduced probability of contamination.